On Tue 02-03-10 20:25:02, Nick Piggin wrote: > On Mon, Mar 01, 2010 at 03:21:49PM -0800, Andrew Morton wrote: > > On Thu, 25 Feb 2010 15:45:58 +0300 > > Dmitry Monakhov <dmonakhov@xxxxxxxxxx> wrote: > > > > > Can someone please describe me why directio deny partial writes. > > > For example if someone try to write 100Mb but file system has less > > > data it return ENOSPC in the middle of block allocation. > > > All allocated blocks will be truncated (it may be 100Mb -4k) end > > > ENOSPC will be returned. As far as i remember direct_io always act > > > like this, but i never asked why? > > > Why do we have to give up all the progress we made? > > > In fact partial writes are possible in case of holes, when we > > > fall back to buffered write. XFS implemented partial writes. > > > > The problem with direct-io writes is that the writes don't necessarily > > complete in file-offset-ascending order. So if we've issued 50 write > > BIOs and then hit an EIO on a BIO then we could have a hunk of > > unwritten data with newly-writted data either side of it. If we get a > > bunch of discontiguous EIO BIOs coming in then the problem gets even > > messier - we have a span of disk which has a random mix of > > correctly-written and not-correctly-written runs of sectors. What do > > we do with that? > > Hmm, what if we're filling in a hole with direct IO? I don't see where > blocks allocated in DIO code will be trimmed on a failed write (because > it's within isize). This could cause uninitalized data of the block to > leak couldn't it? The trick is that blockdev_direct_IO is defined to pass DIO_SKIP_HOLES to __blockdev_direct_IO. Thus e.g. ext2 or ext3 will just fail the direct IO if there is a hole and we fall back to buffered IO which should handle that just fine. Honza -- Jan Kara <jack@xxxxxxx> SUSE Labs, CR -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
